Method in connection with steel production

ABSTRACT

A method of producing a fluxing agent that can be used in production of steel, preferably stainless steel, employs as a raw material a hydroxide sludge that results from neutralization of metal-contaminated pickling liquid from a pickling step for a steel and contains at least one fluoride-containing compound. The hydroxide sludge is calcined. Steel, preferably stainless steel, is produced by decarburizing a steel heat, whereby a slag is formed on top of the steel heat, and adding a fluxing agent to the slag.

TECHNICAL FIELD

The present invention relates to a method of producing a product thatcan be used as a fluxing agent in steel production. The invention alsorelates to a method in connection with steel production, preferably of astainless steel, comprising production of a steel heat, decarburizationof the steel heat whereby a slag is applied on top of said steel heat.Finally, the invention also relates to a product produced according tothe invention.

PRIOR ART

The production of steel, especially stainless steel, comprises annealingand pickling processes. The annealing is a heat treatment operation thataims at recrystallizing the microstructure of the steel and making itductile. In the annealing, an oxide layer is formed on the surface ofthe steel, and a chromium-depleted layer is formed directly beneath theoxide layer. Both of these two layers are removed by pickling.

Pickling means that the annealed steel product is treated by acid, mostoften a mixture of different acids, by which the undesired metaldeposits in the surface are taken away. A mixture of nitric acid, HNO₃,and hydrofluoric acid, HF, is the most efficient for pickling ofstainless steel. The dissolved metals form metal complexes and depositsthat have to be removed from the process. Especially, it is difficult tohandle spent pickling liquids that contain mixed acids, such as amixture of nitric acid (HNO₃) and hydrofluoric acid (HF), containingfluorides. Also the content of e.g. iron, chromium and nickel oxides inthe production of stainless steels, constitutes a handling problem.

After the pickling treatment, the steel product is flushed by water,whereby acidic flushing water is formed. The dissolved metals in theform of metal complexes and deposits, as well as the acidic flushingwater, constitute waste matters of severe environmental impact, and mustbe subjected to special handling in order not to cause severeenvironmental damages. Similarly to the case in other processindustries, there is also a strive within the steel industry to recoverwaste products and to close the cycle.

Several different methods are known to try to regenerate the free acids(HNO₃ and HF) of the spent pickling liquid. A technique for this, whichhas been used for long by the present applicant, is the acid retardationprocess, commonly referred to as SAR (Scanacon Acid Retardation). A SARplant operates to keep the metal concentration in the pickling bath at alow and stable level, and consists of one mechanical and one chemicalprocess step. The mechanical step separates the acid and the metalsludge (metal oxide, metal fluoride), in a solid phase. The chemicalstep separates the acid and dissolved metal ions, by aid of a resin bed.From the SAR plant, a free concentrated acid with a low metal content isrecycled back to the pickling bath. In order to recover yet more freenitric acid an electro-dialysis step can be used that separates anionsand cations in the acid by means of membrane technique. The separationof ions are accelerated by an electrical DC-source. The separated metalions, together with a weak free acid, and the sludge, are pumped to theneutralizing plant for destruction.

Another technique, called the Pyromar process, makes use of thermaldecomposition of metal fluoride complexes in order to recoverhydrofluoric acid, nitric acid and metals. By spray-calcination, a spentpickling liquid is converted to gas phase, where after it can beconverted to a reusable acid by one or more absorption columns. Themetals form metal oxides and must be subjected to reduction before beingused again in the melting shop. The process has several drawbacks. Largeamounts of nitrous fumes (NO_(x)) are formed by the spray-calcination,and these fumes must de destroyed by e.g. selective catalytic removalcontrol (SCR). By the formation of NO_(N), large amounts (about 30-40%)of the nitric acid disappear, which causes an imbalance in the recoveredamounts of hydrofluoric acid and nitric acid. Yet another drawback isthat the metal oxides is in dust form, has a low density (0.5 g/cm³),and that it contains high amounts of fluorides (>1%) that make itdifficult to reduce the oxide product to a metallic form.

Yet another technique is called OPAR (Outokumpu Pickling Acid Recovery),in which sulphuric acid is used to decompose the metal fluoridecomplexes in the spent pickling liquid, by reacting with it and formingmetal sulphates. The mixed acid of HNO₃ and HF, thus recovered, isseparated by evaporation and condensation. The condensate is recycledback to the pickling bath, and metal sulphates formed in the process areheat treated, filter pressed and finally neutralised by calciumhydroxide and spent slag from the melting shop. The process is verycostly and the neutralisation process results in a volume increase of4-5 times, thereby generating large amounts of metal calcium sulphateand metal hydroxide sludge that has to be dumped. No technique forrecycling metal oxides and sulphuric acid exists today.

In the neutralisation plant, the spent pickling liquid is neutralised bycalcium hydroxide, Ca(OH)₂, whereby a sludge results that consists ofdifferent metal hydroxides Me(OH)_(x), calcium fluoride (CaF₂) andcalcium sulphate (CaSO₄). Today, such sludge is dumped.

In case of rain, there is a risk that some metals are leached out fromthe landfill, which means that the leaching water has to be handled andreturned to the neutralisation plant.

During recent years, more stringent environmental demands have amongother things led to stronger demands on landfill designs, which hasresulted in highly elevated costs. Furthermore, a landfill tax may beintroduced in the future. This has led to commenced investigationsconsidering the possibility to keep down the amount of dumped sludge.

In Swedish patent no. SE 519776 of the present applicant, a method ofreutilising metal-containing hydroxide sludge from a pickling step, isdisclosed. The hydroxide sludge is mixed with an admixture having acontent of a substance in group 14 of the periodic table, and is allowedto solidify by hardening or polymerisation, whereby the water contentsinks to below 15%. The solidified mixture may then be recycled to asteel heat in connection with steel production in an arc furnace. Themethod also allows for powdery or finely dispersed residual productscomprising metals, metal oxides and metal hydroxides, to be recycled tothe steel manufacturing. It is also shown that the metals in the productgo into the steel heat, that carbon leaves as carbon dioxide, water aswater vapour (in small amounts), and that silicon, oxides, fluoridesetc. go into the slag. The drawbacks of the method are that fluorideswear on the arc furnace lining, and that water must be driven off whichincreases the processing time in the arc furnace.

It is known from DE 36 34 106 to use a pickling agent distillationresidue containing metal salts, such as a fluoride-containing component,in the production of a slag-forming additive for steel production. Also,a method for production of the slag-forming additive is described, whichcomprises distillation of the pickling liquid in order to drive off thefree acids nitric acid and hydrofluoric acid, and to crystallize metalfluorides as moist sludge. Thereafter, the sludge is filtered in orderto remove additional water and acid, and the dewatered sludge is mixedwith caustic lime, CaO. This mixture can be added to a steel heat as aslag-forming additive. The slag-former will be relatively porous, whichmakes it difficult to handle. Any remaining moist may also cause steamexplosions, and gaseous components will give increased NO_(x) emissionsconstituting a load on the gas cleaning plant of the steel works. Otherdrawbacks of the method are that the distillation process is costly, andthat it is difficult to recycle the nitric acid. About 40% of the nitricacid leaves together with the steam that is driven off, and this nitricacid must be destroyed in the SCR in the gas cleaning plant.

BRIEF ACCOUNT OF THE INVENTION

The invention relates to the handling of hydroxide sludge formed in theneutralisation of spent metal-contaminated pickling agents from apickling step for steel, preferably stainless steel. The controversialidea forms the basis of the invention, that instead of previouslyfocusing on recovery of the metals of the hydroxide sludge, now focusingon the calcium fluoride content, and considering this calcium fluorideto be a resource instead of a load. The present applicant has striven tofind a method that enables handling of the calcium fluoride in thehydroxide sludge, in order to use it as a replacement for naturalfluorspar (commonly called flux) as a fluxing agent. This is achieved bya method of producing a fluxing agent that can be used in production ofsteel, preferably stainless steel, characterised in that as a rawmaterial for the production of said fluxing agent is used a hydroxidesludge resulting from neutralisation of metal-contaminated picklingliquid from a pickling step for a steel, said hydroxide sludgecontaining at least one fluoride-containing compound, and that saidhydroxide sludge is calcined. The invention also provides a method inconnection with steel production, preferably stainless steel, comprisingproduction of a steel heat and decarburization of the steel heat,whereby a slag is formed on top of said steel heat, characterised inthat a product according to the invention is added to said slag.

By the invention, it is also possible to achieve one or some of thefollowing advantages:

-   -   hydroxide sludge can be recycled to the steel production        essentially without any process drawbacks    -   hydroxide sludge can be recycled to the steel production        essentially without any health hazards to the personnel    -   the metals in the hydroxide sludge can be recovered    -   hydroxide sludge may replace natural fluorspar essentially        without impairing the properties of the produced steel    -   hydroxide sludge already dumped and originating from acidic,        metal-contaminated pickling liquids, can be taken care of    -   hydroxide sludge can be processed into a mechanically stable        product that may constitute a fluxing agent    -   a fluxing agent can be produced by a method that in its        essentials is simple and cost-efficient    -   a fluxing agent can be produced essentially without health        hazards for the personnel

The invention has been developed primarily for use in connection withthe production of stainless steels, but it can also be used inconnection with other types of steel production, such as production ofcarbon steel.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS

FIG. 1 is a flowsheet for the process,

FIG. 2 shows a graph over the contents of Cr⁶⁺ in an exhaust emissioncontrol plant.

DETAILED DESCRIPTION OF THE INVENTION

The inventive process is described below with reference to the flowsheetin FIG. 1.

Acidic, metal-contaminated and spent pickling liquids that can behandled according to the invention are chemically acidic picklingliquids 1 as well as neutral pickling liquids for electrolytic pickling.Such spent pickling liquids comprise residual acids, such ashydrofluoric acid, nitric acid, sulphuric acid, salts of such acids,including sodium sulphate e.g., and dissolved metal fluorides and metaloxides. In connection with the method, a per se known neutralisation 2of the spent pickling liquid is performed to a pH of about 9-10, byaddition of alkali, usually milk of lime, Ca(OH)₂, but also otheralkaline additives may be used, for example CaCO₃, NaOH. Prior to theneutralisation chromium reduction of the liquid from the neolytepickling step 9 (pH 6-6.5) may take place 3. Regeneration of free acidsin the pickling acids is performed in a SAR-step 4 and electro-dialysisstep 5. Reduction of nitrous fumes (NO_(x)) can be obtained by selectivecatalytic removal control (SCR) or hydrogen peroxide treatment 10.

After the neutralisation, the neutralised pickling liquid is dewatered6, suitably mechanically in a filter press e.g., to a dry substancecontent of at least 30% by weight, preferably at least 50% by weight andup to 80% by weight, but normally not more than up to 70% by weight.Effluent water containing some nitrates (Ca(NO₃)₂, pH 9-10) can be ledto the recipient.

The dewatered product is called hydroxide sludge. The hydroxide sludgecontains e.g. CaF₂, CaSO₄ and Fe-, Cr-, Ni-hydroxides and Ca- orFe-molybdate, at least in the case that hydrofluoric acid is used in thepickling liquid and in the case of stainless steel production. Thehydroxide sludge is dried and calcined or sintered 7 to a mechanicalstable product before it is added as a fluxing agent in theAOD-converter 8 in the steel production.

With the purpose of finding suitable methods for treatment of thehydroxide sludge, extensive experiments have been conducted in order toobtain a product capable of constituting a fluxing agent.

Undertaken Experiments

Introductory experiments for the treatment of hydroxide sludge havecomprised:

-   -   calcination and melting in a 3 kg Tamman furnace (>1500° C.),    -   melting in a 20 kg rotating furnace,    -   calcination in a small scale Kanthal furnace (1100° C.),    -   calcination in a bell-type furnace, 500 kg/heat,    -   calcination in a pilot scale Kaldo converter (6 tons), by aid of        a LPG burner,    -   drying in a drying chamber (200° C.),    -   treatment in a pilot scale DC furnace (10 tons and 3 MW).

Of these experiments, it is the calcination experiments in anelectrically heated, stationary bell-type furnace and the calcinationexperiments in an LPG heated, rotating Kaldo converter that will bedescribed further. By these experiments, a well sintered, mechanicallystable and dustless product has been achieved, which product has thenbeen used in subsequent experiments for evaluation of the properties ofthe calcined sludge as a fluxing agent in a converter instead of or incombination with regular fluorspar.

In a preferred embodiment of the invention, an electrically heated,stationary furnace (of bell-type) is used for the calcination ofhydroxide sludge. From the description below, it is evident that thisresults in a product having very good properties in terms of mechanicalstability, sintering degree and piece/particle size, but according toundertaken pilot experiments, a Kaldo converter can also be used for thecalcination. Furthermore, undertaken experiments of calcining in arotary kiln have shown to give a stable product. These experiments willnot be described further since the hydroxide sludge had an exceptionalhigh moisture content of 55-60% which resulted in abnormal problemsduring the calcining. However, one conclusion from the experiment isthat the fines fraction is likely to be reduced if the moisture contentin the hydroxide sludge and the rotating speed of the rotary kiln aresufficiently low.

The process for calcination of hydroxide sludge in a stationary furnace(bell-type) or rotary kiln which is electrically heated or heated byLP-gas or oil, can be described according to the following:

-   -   heating to 150-200° C., evaporation of free water in the sludge    -   heating to 600-900° C., evaporation of crystal water, whereby        the material becomes completely dehydrated    -   heating to 1000-1200° C., sintering to a mechanically stable        product (hydroflux)

The process for calcination of hydroxide sludge in a converter (Kaldoconverter type) which is LPG heated, can be described according to thefollowing:

a) heating to 150-200° C., evaporation of free water in the hydroxidesludgeb) heating to 600-900° C., chemically bonded water leavesc) heating to 1200-1300° C., the hydroxide sludge meltsd) discharging the molten hydroxide sludge from the furnace to cool downto ambient temperature during solidification to form a mechanicallystable product (hydroflux)e) crushing of the solidified hydroflux product

Pilot Experiments

For the calcination was used 10 tons of hydroxide sludge having arelatively high sulphur content, and 2 tons of hydroxide sludge having arelatively low sulphur content. The high sulphur content hydroxidesludge was split into 3 portions that were calcined in a Kaldo converterafter first having been dried in a chamber furnace at 200° C. Portions 1and 2 were heated to 900° C. in the Kaldo converter, and portion 3 wasmelted in the Kaldo converter. The low sulphur content hydroxide sludgewas split into two portions, whereof one portion, portion no. 4, wasdried in a chamber furnace according to the above, and the otherportion, portion no. 5, was air dried at ambient temperature. These twolow sulphur content portions were calcined in a bell-type furnace.

The average composition of the two categories of hydroxide sludge at 85%dry content, i.e. after drying at 110° C. but before calcination, areshown in the table below.

TABLE 1 CATEGORY Fe(OH)₃ Cr(OH)₃ Ni(OH)₂ MoO₃ CaF₂ CaSO₄ C Na₂O CaO Highsulphur content 23.4 10.4 3.2 0.3 44.2 3.2 0.95 0.1 9.2 Low sulphurcontent 24.1 5.1 3.6 0.3 43.6 0.3 0.5 0 8.5

In the table below, properties and amounts of the calcined hydroxidesludge that was obtained, and the temperature during calcination for therespective portions, are shown.

TABLE 2 TEMPERATURE PORTION WEIGHT (KG) (° C.) PROPERTIES 1 Totally 1850 900 small, loosely sintered 2 lumps, 10-20 mm, of poor strength 3 4501300 Hard, molten lumps, must be crushed before use 4 Totally 670 1000-1100 Hard, sintered lumps 5 having a size of 10-40 mm

During calcination, the carbon content was decreased by carbon leavingas carbon dioxide. The carbon originates from calcium carbonate thataccompanies the lime in the neutralisation process. The materials fromportions no. 1 and 2 were agglomerated, but had a very poor strength andsmall size. The material from portion no. 3 was very much similar tonatural fluorspar, in respect of strength and size.

During calcination in the bell-type furnace, it was noted that thecarbon content was low to even in this case. The material from portionsno. 4 and 5 had very good strength properties, in practice equally goodas the molten material from portion no. 3.

The materials from portions no. 1 and 2 were judged unsuitable forcontinued experiments, due to the risk of dusting and due to poorstrength. The materials from portions no. 3, 4 and 5 had none or a verysmall ratio of fines fraction, and were judged suitable to be used insubsequent experiments. The composition of these materials is shown inthe table below (% by weight):

TABLE 3 PORTION Fe₂O₃ Cr₂O₃ NiO MoO₃ CaF₂ CaSO₄ C Na₂O CaO SiO₂ MgO 125.2 11.8 3.1 0.3 47.8 3.0 0.01 0.1  7.1 1.8 0.6 2 25.4 11.6 3.0 0.247.6 2.9 0.01 0.1  7.3 1.9 0.6 3 25.3 11.7 3.0 0.2 41.0 2.8 0.01 0.114*) 2.0 0.5 4 27.9 6.1 3.4 0.3 51.5 0.4 0.02 0 10.5 1.9 0.4 5 27.6 5.93.5 0.2 51.2 0.4 0.02 0 10.8 2.1 0.4 *)Given as residual content up to100%

During the experiments, no measurements were made in respect ofoccurrence of HF or SO₂, both of which are irritating already at lowcontents. The table shows however, concerning portion no. 3, that CaF₂is partially lost at temperatures close to 1300° C. No loss of CaF₂ hasbeen noted concerning portions no. 1, 2, 4 and 5.

Natural fluorspar normally used in the melting shop, has the followingapproximate composition (% by weight):

TABLE 4 Composition of natural fluorspar PORTION CaF₂ SiO₂ CaCO₃ MgOFe₂O₃ S K₂O Pb P Fluorspar ≧90 7.5-8 ≦0.5 ~0.05 ~0.2 ~0.03 ~0.02 ≦0.01≦0.01

In principle, the calcined hydroxide sludge is a chemically producedsynthetic fluorspar, although having a maintained content of metaloxides and a small surplus of calcium oxide. In the following, theproduct is called hydroflux. The subsequent experiments aimed atinvestigating the properties of the hydroflux as a fluxing agent. Asecondary purpose was to investigate whether the metal oxides would beallowed to be reduced into the steel heat.

Nine heats of 6 tons each and being of a stainless steel of the typeASTM 304 were produced in an arc furnace, for the pilot experiments. Therespective heats were tapped into a heated tapping ladle, and weretransported to a 6 ton AOD-converter for decarburization. The amount ofslag accompanying from the arc furnace, was minimised. Before theexperiments, the AOD-converter in question was provided with a newlining, in order thereby to be able to decide whether the hydroflux hadan influence on the same.

The nine heats constituted test materials for an experimental campaignthat was run in series in the AOD-converter. The experiments wereconducted with varying mixing ratios of natural fluorspar and hydroflux.In five experiments, hydroflux from portions no. 4 and 5 were used,since this hydroflux had lower sulphur content, but two experiments withhydroflux from portion no. 3 were also included in the experimentalcampaign. Two reference heats with solely natural fluorspar were run. Inorder to reduce the metal oxides, primarily Cr₂O₃ but also Fe₂O₃, andNiO, it was found that an extra addition of FeSi was needed. The amountof FeSi required for the metal reduction is proportional to the metaloxides in the hydroflux. Moreover, an extra addition of CaO is requiredto maintain the basicity of the slag, which should be about 1.5-2.0.These extra additives result in that the amount of slag increases withup to 10%.

In the pickling, different types of steel result in different amounts ofmetal hydroxides in the sludge. One advantage of the invention is thatyou are not bound to use hydroxide sludge from the pickling of the sameor essentially the same type of steel as the one you intend to produce.Therefore, the skilled person will realise that the invention is veryeasy to integrate in the existing steel production process. Thanks tothe invention, it is also possible to handle already dumped hydroxidesludge, converting it to a valuable fluxing agent, and recovering itsmetal content.

The mixing ratios of the respective heats of the pilot experimentcampaign are given in the table below. The numbers in parenthesis relateto the portion in question of the hydroflux:

TABLE 5 Mixing ratio of natural fluorspar and hydroflux Heat no. CaF₂from natural fluorspar CaF₂ from hydroflux 1 100 0 2 50 50 (4 + 5) 3 5050 (4 + 5) 4 60 40 (4 + 5) 5 50 50 (4 + 5) 6 25 75 (4 + 5) 7 65 35 (3) 8100 0 9 50 50 (3)

The AOD-process may be described according to the following:

-   -   Charging of 6 tons of steel from a tapping ladle    -   Temperature measurement directly after charging    -   Lollipop and chill mould testing for analysis of carbon content        and steel composition

Decarburization:

-   -   Addition of lime and dolomite just after commenced oxygen        blowing, guideline value C=0.40%    -   Addition of lime, continued oxygen blowing and also nitrogen        gas, guideline value C=0.15%    -   Continued oxygen and nitrogen gas blowing, guideline value        C=0.07%. Sampling for carbon content and temperature.

Optional addition of cooling scrap. Sampling for carbon content andtemperature.

Reduction:

-   -   Addition of hydroflux, lime, FeSi, fluxing agents (natural        fluorspar), SiMn, optionally cooling scrap. Argon gas blowing        for stirring.    -   Sampling of steel, slag and temperature    -   Tapping

In addition to the above described process steps, a continuousmeasurement of gas flows to the AOD-converter (O₂, N₂), exhaust gasflows from the AOD-converter (CO, CO₂, O₂), and material weight todifferent hoppers, took place. In order to investigate whether the useof hydroflux generates additional Cr⁶⁺, the content of Cr⁶⁺ was analysedin the water pool of a venturi scrubber in an exhaust emission controlplant, see FIG. 2. The content was analysed after the respectiveexperimental heats. The arrows indicate the reference heats withaddition solely of natural fluorspar. From these measurements it isclear that the use of hydroflux does not give rise to increased Cr⁶⁺formation, compared to the use of natural fluorspar, which is seen inFIG. 2.

The slag samples from the 9 experimental heats were analysed and theircompositions are given in the table below.

TABLE 6 Composition of slag samples from the 9 experimental heats AOD 1AOD 2 AOD 3 AOD 4 AOD 5 AOD 6 AOD 7 AOD 8 AOD 9 Cr₂O₃ 5.0 3.6 1.1 1.00.2 0.2 0.3 0.4 0.1 FeO 0.4 0.6 1.3 0.3 0.2 0.3 0.4 0.6 0.3 MgO 3.7 4.37.4 10.6 9.1 8.6 10.3 10.2 10.1 SiO₂ 32.7 31.6 32.6 30.4 30.4 29.6 29.631.5 30.4 CaO 54.7 57.6 56.2 56.4 62.7 62.9 60.2 57.5 61.8 F 4.6 4.4 4.24.5 3.9 5.0 4.1 3.6 3.5 CaF₂ 9.5 9.0 8.6 9.2 8.0 10.2 8.4 7.4 7.2 CaOcalc 47.9 51.1 50.2 49.8 56.9 55.5 54.2 52.2 56.6 MnO 1.2 0.9 0.4 0.40.1 0.1 0.1 0.2 0.1 Al₂O₃ 0.7 0.9 1.4 2.4 1.6 1.6 2.7 2.5 1.9 CaSO₄ <0.3<0.3 <0.3 <0.3 <0.3 <0.3 2.7 <0.3 2.7 Basicitet 1.5 1.6 1.5 1.6 1.8 1.91.8 1.7 1.9

It is clear from the table that a few introductory experimental heatswere required to find the optimum conditions for reduction of thechromium oxide. During these introductory experimental heats, therequired amount of extra FeSi was found. Experimental heats no. 7 and 9were produced with sulphuric containing hydroflux, and in these cases asmall increase of the sulphur content in the slag could be noted. Ananalysis of the sulphur content of the steel indicates that sulphur isnot re-introduced into the heat, and therefore one may assume that asmall part of the sulphur ends up in the slag and the main part leaveswith the exhaust gases as SO₂.

Experience from the conducted pilot experiments surprisingly shows thatthe hydroflux is also suitable for use as a fluxing agent in connectionwith carbon steel production. In connection with carbon steelproduction, the use of fluorspar has, as is well known, mainly beenterminated. Instead, lime and iron oxide is used. The applicant wouldall the same point out the option of using hydroflux in theseapplications. In such a process, it is not necessary to reduce the metaloxides in the slag, why the extra addition of FeSi and CaO can beomitted. The purpose of CaF₂ in the hydroflux is also in this case torender the slag fluid.

As a conclusion, the pilot experiments have shown that:

-   -   Hydroxide sludge can be calcined into a mechanically stable        product, suitable for use as a fluxing agent in an        AOD-converter. No dusting has been observed.    -   The product, so called hydroflux, can be produced by a simple        and cost-efficient method    -   The use of hydroflux has not shown any negative effects on the        reduction process in the AOD-converter    -   The formation of slag by aid of hydroflux is equivalent with the        one aided by natural fluorspar, and the slag has a good        reactivity    -   The reduction slag in the AOD-converter had essentially the same        properties independent of if hydroflux or natural fluorspar was        used, among other things a low viscosity and the same colour.    -   The extra metal oxides in the hydroflux may be efficiently        reintroduced into the heat by addition of FeSi.    -   No uptake of S and C in the heat could be observed at the use of        hydroflux.

Full-Scale Experiments

Three sets of full scale experiments have been performed:

-   -   1. Initial test campaign. Screened, low sulphur hydroflux added        to the AOD-converter in tilted position    -   2. 2^(nd) test campaign. Screened, high sulphur hydroflux added        to the AOD-converter in upright position    -   3. 3^(rd) test campaign. Unscreened, high sulphur hydroflux        added to the AOD-converter in upright position

Initial Test Campaign

For full scale experiments, four heats were produced in an arc furnace,each of about 90 tons in a sequence of a steel grade called ASTM 304L,i.e. of type ASTM 304 with low carbon content. By “sequence” is meant aproduction of several heats with the same steel code, after each other.The first heat of the sequence is an experimental heat, and the rest arereference heats. Routine samples of the steel's initial composition weretaken from the arc furnace in the transfer ladle, before the de-slaggedsteel was discharged into an AOD-converter and the decarburization wascommenced. After the decarburization with oxygen and argon in theAOD-converter, the reduction was commenced.

Hydroflux corresponding to about 40% of the required CaF₂ was added froma box that usually is used for cooling scrap. The hydroflux had aparticle size of at least 12 mm, without any fines, and was wellsintered and mechanically stable. The addition was made as a first stepin the reduction, and with the AOD-converter in tilted position.Thereafter, the converter was raised to operating position, and the restof the reduction mixture was added from the hoppers of alloying elementsabove the converter. The composition of the used hydroflux is shown inthe table below.

TABLE 7 Composition of the hydroflux (% by weight) used in the initialcampaign CaF₂ 51 Fe₂O₃ 27 Cr₂O₃ 5 NiO 4 CaSO₄ 0.3 SiO₂ 2 S 0.06 C 0.01

In order to form a reducing slag, the following additions were made tothe AOD-converter during the reduction step:

TABLE 8 Additives to the reduction step (kg) Heat no. Test heatReference 1 Reference 2 Reference 3 FeSi, (Si, 75%) 1302 1063 1653 1331FeSiMn 1687 1702 1699 1412 Burnt lime (CaO) 1535 1375 2814 1752Fluorspar 750 983 1333 1140 (CaF₂, 90%) Hydroflux 780 — — — (CaF₂, 51%)

The following amounts of steel (tons) have been charged into anddischarged from the converter:

TABLE 9 Steel amounts in the AOD-converter (tons) Test heat Reference 1Reference 2 Reference 3 Initial Weight 92.0 87 83 86.1 Final Weight101.5 99.7 100.2 94.6Results from the Production of the Four Sequence Heats

The production of the four sequence heats followed a normal course.Normal production samples and extra slag samples were taken from thereduction step. Corresponding samples were taken from the threereference heats. The analyses of the steel samples after reduction areshown in table 10. The slag samples from the reduction are shown inTable 11.

TABLE 10 Steel samples (ASTM 304L) from the AOD-converter afterreduction Element Test heat Reference 1 Reference 2 Reference 3 C 0.0150.013 0.016 0.017 Si 0.30 0.19 0.22 0.12 Mn 1.65 1.61 1.46 1.53 P 0.0230.023 0.024 0.021 S 0.010 0.009 0.013 0.011 Cr 18.18 18.00 17.72 17.91Ni 8.17 8.05 8.13 8.09 Mo 0.34 0.31 0.32 0.41 Cu 0.25 0.22 0.22 0.20 N0.092 0.077 0.079 0.077

TABLE 11 Slag samples after reduction (% by weight) Element Test heatReference 1 Reference 2 Reference 3 SiO₂ 35.4 35.6 37.2 36.7 MnO 1.2 1.22.4 1.4 P₂O₅ 0.0 0.0 0.0 0.0 S 0.04 0.03 0.01 0.05 Cr₂O₃ 1.0 1.1 2.1 1.3NiO 0.05 0.04 0.04 0.04 TiO₂ 0.30 0.26 0.36 0.33 Al₂O₃ 1.6 1.0 0.8 1.4V₂O₅ 0.01 0.01 0.02 0.02 MgO 5.2 6.1 5.3 5.6 CaO 55.7 55.8 52.7 54.9CaO, Calc. 49.9 50.8 47.5 49.0 FeO 0.3 0.2 0.3 0.3 CaF₂ 8.1 6.9 7.1 7.4Basicity 1.4 1.4 1.3 1.4

The addition of hydroflux in the initial campaign worked according tothe expectations, and the observations can be summarised as follows:

-   -   No dusting or strong reaction took place at the addition of the        hydroflux    -   The appearance of the slag was similar, and it was well fluid        for all four heats    -   The content of Cr₂O₃ in the reduction slag was equal in all        heats    -   No changing of properties for the ladle furnace slag were        observed before tapping into the tundish in the continuous        casting plant for the test heat in comparison with the reference        heats.

Results of the Evaluation of the Final Material

The test heat and the reference heats have been compared in respect ofthe quality of the materials. 12 steel strips were made from thematerial, and the qualities of these were investigated according to thefollowing:

-   -   Investigation of the weldability of the strips    -   Investigation of slag inclusions in the strips    -   Establishing strength values    -   Manual inspection of the surface quality in respect of surface        defects

Comparison of Weldability

Comparative welding tests have been performed by certified weldersaccording to a method called MMA/SMAW, using an electrode type called308L/MVR AC/DC. No visual difference could be seen in respect ofweldability, such as flow and slag release, when comparing test andreference heats.

Comparison of Slag Inclusions in the Materials

All together, 6 samples have been evaluated in respect of the occurrenceof slag inclusions. 3 samples were taken from the test heat and 1 sampleeach was taken from the reference heats. The samples have been analysedin a PC controlled metal microscope at Avesta Research Centre (ARC) andaccording to regulation SS 111116, i.e. the method that the ARC uses forroutine control of slag inclusions in stainless materials. All 6 samplesshow low occurrence of slag inclusions.

Strength Analysis and Manual Inspection of Surface Quality

The strength tests were approved for all 12 strips. The surfaceinspection revealed no deviations as compared to other strips producedduring the same period in the strip mill in question.

2^(nd) Test Campaign

Twenty-two heats in three sequences of the steel grades called ASTM 304Land ASTM 316L were produced. Seven reference heats were included in thesequence. A total of 19 tons of hydroflux was added to the heats invarious amounts. 50-100% out of the total requirement of fluxing agentin each heat respectively was provided by addition of hydroflux whichcan be seen in table 13 below. Three reference heats were produced withnatural fluorspar as the only fluxing agent. The hydroflux was added ina conventional manner to the AOD-converter in upright position from ahopper above the converter.

The hydroflux used was produced in a stationary electric furnace wherethe hydroxide sludge was dried and calcined/sintered at a finaltemperature of 1050° C. The hydroflux was screened to a particle sizeabove 4 mm. The hydroxide sludge used originated from four differentneutralisation plants that was mixed. In this way the hydroflux producedobtained realistic variations in its compositions depending on thatparticular mixture. The composition of the hydroflux varied within theranges shown in table 12 below.

TABLE 12 Variations in the composition of the used hydroflux (% byweight) in the test campaign CaF₂ 40-60 Fe₂O₃ 22-30 Cr₂O₃ 5-8 NiO 3-5CaO  2-20 SiO₂ 1.5-2   CaSO₄ 0.5-14  C 0.01-0.02

TABLE 13 Additives (kg) to the heats in the test campaign Heat Steelgrade no. CaF₂ kg CaO kg 75 FeSi FeSiMn Hydroflux 2323 844117 1006 8561723 0 0 2323 844118 479 854 1610 0 1027 2323 844119 517 1704 2125 01040 2323 844120 502 1760 2216 0 1047 2323 844121 516 1552 2200 0 10432323 844122 1002 1697 2051 0 0 1358 844123 1021 1118 1175 1402 0 1358844124 521 1321 1332 1354 1037 1358 844125 523 937 1302 1227 1027 1358844126 214 1509 1381 1321 1530 1358 844127 208 1405 1218 1243 1520 1358844128 0 2500 1556 1122 2017 1358 844129 0 946 1071 1335 1832 1358844130 986 1252 1297 1252 0 2323 844167 258 1859 2324 0 1515 2323 844168262 1168 2027 0 1448 2323 844169 981 1083 1786 0 0 1358 844172 1219 27661467 1054 0 1358 844173 631 2032 1469 1362 820 1358 844174 535 2105 11001478 1200 1358 844175 547 1300 901 1736 994 1358 844176 965 766 826 18870Results from the Production of the Nineteen Sequence Heats

The production of the nineteen sequence heats followed a normal course.Normal production samples and extra slag samples were taken from thereduction step. Corresponding samples were taken from the sevenreference heats. The analyses of the steel samples after reduction areshown in table 14, 16 and 18 and the slag samples from the reduction areshown in Table 15, 17 and 19.

TABLE 14 Steel samples (ASTM 316L) from the AOD-converter afterreduction Heat 844117 844118 8488119 844120 844121 844122 844167 844168844169 Element REF 4 Test 2 Test 3 Test 4 Test 5 REF 5 Test 6 Test 7 REF6 C 0.019 0.015 0.014 0.016 0.012 0.013 0.011 0.012 0.012 Si 0.40 0.470.55 0.43 0.35 0.39 0.42 0.43 0.46 Mn 0.91 0.84 0.85 0.84 0.97 0.93 1.030.99 0.89 P 0.026 0.023 0.024 0.024 0.027 0.026 0.026 0.026 0.026 S0.007 0.006 0.008 0.009 0.012 0.008 0.005*⁾ 0.007*⁾ 0.004 Cr 16.92 16.7216.66 16.64 16.89 16.70 16.73 16.84 16.92 Ni 10.15 10.16 10.19 10.1610.36 10.16 10.09 10.12 10.06 Mo 1.94 2.00 2.04 2.00 2.10 2.03 2.18 2.001.96 Cu 0.45 0.33 0.33 0.34 0.42 0.44 0.34 0.37 0.35 N 0.053 0.039 0.0520.058 0.053 0.053 0.040 0.055 0.053 *)≈75% out of the total requirementof fluxing agent added as hydroflux with 4-6% CaSO₄ doesn't effect thesulphur content in the steel

The hydroflux was added in connection to the reduction step. In order toevaluate the efficiency of the fluxing agent the amounts of Cr₂O₃remaining in the slag was measured. A well working reduction produces aslag where the amount of Cr₂O₃ remaining in the slag is max. 1.0%.

TABLE 15 Slag samples after reduction ASTM 316L (% by weight) Heat844117 844118 8488119 844120 844121 844122 844167 844168 844169 ElementREF 4 Test 2 Test 3 Test 4 Test 5 REF 5 Test 6 Test 7 REF 6 S 0.20 0.150.13 0.11 0.13 0.16 0.13 0.14 0.12 F 4.4 3.6 4.5 3.8 4.1 4.2 4.3 3.9 5.1MnO 0.2 0.2 0.2 0.3 0.3 0.2 0.3 0.4 0.2 P2O5 0 0 0 0 0 0 0 0 0 Cr2O3 0.70.5 0.7 0.6 0.5 0.5 0.4 0.4 0.3 NiO 0.04 0.04 0.04 0.05 0.04 0.04 0.040.04 0.05 TiO2 0.33 0.22 0.17 0.19 0.18 0.27 0.19 0.28 0.25 V2O5 0.010.01 0.02 0.01 0.02 0.03 0.02 0.01 0.01 Al2O3 1.7 1.2 1.2 1.2 1.2 1.31.0 1.1 1.1 CaF2 9.1 7.3 9.1 7.7 8.4 8.5 8.7 8.0 10.5 CaO 59.5 58.1 59.058.5 57.6 58.7 59.0 57.4 59.2 CaO ber 53.0 52.8 52.4 53.0 51.6 52.6 52.751.6 51.7 FeO 0.1 0.1 0.2 0.2 0.2 0.1 0.2 0.7 0.3 MgO 5.8 6.4 5.3 5.15.6 5.6 7.6 6.4 6.6 Basicity 1.6 1.5 1.6 1.5 1.6 1.6 1.6 1.5 1.6

TABLE 16 Steel samples (ASTM 304L) from the AOD-converter afterreduction Heat 844123 844124 844125 844126 844127 844128 844129 844130Element REF 7 Test 8 Test 9 Test 10 Test 11 Test 12 Test 13 REF 8 C0.020 0.015 0.015 0.015 0.013 0.020 0.010 0.015 Si 0.36 0.19 0.26 0.280.26 0.27 0.21 0.20 Mn 1.62 1.67 1.66 1.68 1.68 1.57 1.65 1.67 P 0.0230.027 0.029 0.029 0.029 0.028 0.029 0.026 S 0.006 0.006 0.011 0.0070.006 0.015*) 0.011*) 0.010 Cr 17.88 17.85 17.95 18.08 17.97 17.77 17.8618.00 Ni 8.17 8.08 8.09 8.09 8.05 8.09 8.03 8.05 Mo 0.36 0.36 0.38 0.370.42 0.37 0.36 0.36 Cu 0.18 0.20 0.33 0.30 0.30 0.30 0.31 0.29 N 0.0790.073 0.085 0.079 0.074 0.068 0.083 0.081 *)100% out of the totalrequirement of fluxing agent added as hydroflux with 4-6% CaSO₄ has amarginal effect on the sulphur content in the steel

TABLE 17 Slag samples after reduction ASTM 304L (% by weight) Heat844123 844124 8488125 844126 844127 844128 844129 844130 Element REF 4Test 2 Test 3 Test 4 Test 5 REF 5 Test 6 Test 7 S 0.070 0.10 0.27 0.280.23 0.04 0.17 0.13 F 4.4 4.3 4.9 5.5 5.3 4.1 4.1 3.4 SiO2 33.2 33.232.5 31.9 31.6 34.3 32.3 34.3 MnO 0.4 0.5 0.5 0.3 0.3 1.1 0.6 0.6 P2O5 00 0 0 0 0 0 0 Cr2O3 0.4 0.4 0.3 0.2 0.2 1.0 0.4 0.5 NiO 0.04 0.04 0.040.04 0.04 0.04 0.04 0.04 TiO2 0.37 0.24 0.23 0.25 0.27 0.30 0.26 0.30V2O5 0.01 0.01 0.01 0.01 0.01 0.02 0.01 0.01 Al2O3 0.9 0.9 0.9 1.0 1.01.1 0.9 1.1 CaF2 9.0 8.7 10.0 11.3 10.9 8.4 8.3 6.9 CaO 58.2 57.7 57.558.8 58.8 54.8 58.4 57.2 CaO calc 51.7 51.4 50.3 50.7 51.0 48.8 52.452.2 FeO 0.1 0.1 0.2 0.1 0.1 0.4 0.2 0.1 MgO 6.7 7.3 7.7 7.6 8.3 6.4 7.96.5 Basicity 1.6 1.5 1.5 1.6 1.6 1.4 1.6 1.5

TABLE 18 Steel samples (ASTM 304L) from the AOD-converter afterreduction Heat Ele- 844172 844173 844174 844175 844176 ment REF 9 Test14 Test 15 Test 16 REF 10 C 0.012 0.013 0.014 0.016 0.014 Si 0.17 0.180.19 0.15 0.27 Mn 1.57 1.59 1.64 1.66 1.65 P 0.025 0.028 0.025 0.0230.023 S 0.008 0.015*⁾ 0.020*⁾ 0.015*⁾ 0.011 Cr 17.83 17.97 17.95 18.1118.04 Ni 8.14 8.00 8.10 8.15 8.12 Mo 0.58 0.32 0.37 0.34 0.37 Cu 0.320.29 0.34 0.32 0.32 N 0.069 0.070 0.070 0.085 0.069 *)45-56% out of thetotal requirement of fluxing agent added as hydroflux with 12-14% CaSO₄has a marginal effect on the sulphur content in the steel

TABLE 19 Slag samples after reduction ASTM 304L (% by weight) Heat844172 844173 844174 844175 844176 Element REF 9 Test 14 Test 15 Test 16REF 10 S 0.070 0.10 0.27 0.28 0.23 F 4.4 4.3 4.9 5.5 5.3 SiO2 33.2 33.232.5 31.9 31.6 MnO 0.4 0.5 0.5 0.3 0.3 P2O5 0 0 0 0 0 Cr2O3 0.4 0.4 0.30.2 0.2 NiO 0.04 0.04 0.04 0.04 0.04 TiO2 0.37 0.24 0.23 0.25 0.27 V2O50.01 0.01 0.01 0.01 0.01 Al2O3 0.9 0.9 0.9 1.0 1.0 CaF2 9.0 8.7 10.011.3 10.9 CaO 58.2 57.7 57.5 58.8 58.8 CaO calc 51.7 51.4 50.3 50.7 51.0FeO 0.1 0.1 0.2 0.1 0.1 MgO 6.7 7.3 7.7 7.6 8.3 Basicity 1.6 1.5 1.5 1.61.6

Given the experiments performed hitherto, it seems that sulphur in thehydroflux does not significantly affect the steel production process.The main part of the sulphur leaves together with the effluent gas fromthe AOD-converter as SO₂.

The addition of hydroflux in the 2^(nd) test campaign worked accordingto the expectations, and the observations can be summarised as follows:

-   -   The hydroflux could be transported to the hoppers without any        negative influence on the mechanical properties    -   The addition of hydroflux from the hoppers worked without any        problems    -   No dusting took place at the addition of the hydroflux    -   The appearance of the slag was similar, and it was well fluid        for all heats    -   Variations within the content of CaF₂ didn't effect on the        properties of the slag    -   The content of Cr₂O₃ in the reduction slag was equally low in        all heats    -   The basicity of the slag where the same (1.4-1.6) for all heats    -   Natural fluorspar could be completely substituted to hydroflux        without any negative effects    -   Hydroflux with 14% CaSO₄ did not effect the sulphur content in        the steel    -   Hydroflux gave a somewhat more fluid slag than natural fluorspar    -   No changing of properties for the ladle furnace slag were        observed before tapping into tundish in the continuous casting        plant

3^(rd) Test Campaign

Unscreened, high sulphur hydroflux added to the AOD-converter in uprightposition. Nine heats in a sequence of the steel grades called ASTM 304Lwere produced. Three reference heats were included with naturalfluorspar as the only fluxing agent. A total of 7 tons of unscreened,high sulphur hydroflux was added into the six test heats in amounts of50-75% out of the total requirement of fluxing agent in each heatrespectively. The hydroflux was added in a conventional manner to theAOD-converter in upright position from a hopper above the converter. Thehydroflux contained approximately 20% fines with particle sizes smallerthan 4 mm. The composition of the hydroflux varied within the rangesshown in table 20 below.

TABLE 20 Variations in the composition of the used hydroflux (% byweight) in the test campaign CaF₂ 45-60 Fe₂O₃ 24-30 Cr₂O₃ 5-9 NiO 3-5CaO  2-24 SiO₂ 1.5-2   CaSO₄ 5-7 C 0.01-0.02

In order to form a reducing slag, the following additions were made tothe AOD-converter during the reduction step in the campaign:

TABLE 21 Additives (kg) to the heats in the test campaign Steel gradeHeat no CaF₂ kg CaO kg 75 FeSi FeSiMn Hydroflux 1358 451132 1000 800 8501380 0 1358 451133 274 1492 1304 1518 1069 1358 451134 622 809 863 13871045 1358 451135 614 722 1196 1223 1060 1358 451136 1022 815 1029 1261 01358 451137 522 1361 1294 1396 1069 1358 451138 363 1557 1379 1520 15471358 451139 372 1482 1460 1276 1541 1358 451140 1027 1936 1574 1149 0

TABLE 22 Steel samples (ASTM 304L) from the AOD-converter afterreduction Heat 451132 851133 851134 851135 851136 851137 851138 851139851140 Element REF 11 Test 17 Test 18 Test 19 REF 12 Test 20 Test 21Test 22 REF 13 C 0.012 0.013 0.014 0.016 0.014 0.018 0.018 0.017 0.016Si 0.17 0.18 0.19 0.15 0.27 0.39 0.14 0.16 0.25 Mn 1.57 1.59 1.64 1.661.65 1.61 1.57 1.61 1.53 P 0.025 0.028 0.025 0.023 0.023 0.028 0.0260.027 0.026 S 0.008 0.015*⁾ 0.020*⁾ 0.015*⁾ 0.011 0.008 0.012*) 0.0050.004 Cr 17.83 17.97 17.95 18.11 18.04 18.07 18.29 18.03 17.92 Ni 8.148.00 8.10 8.15 8.12 8.16 8.24 8.13 8.01 Mo 0.58 0.32 0.37 0.34 0.37 0.360.36 0.37 0.31 Cu 0.32 0.29 0.34 0.32 0.32 0.36 0.43 0.47 0.31 N 0.0690.070 0.070 0.085 0.069 0.077 0.084 0.079 0.079 *)70% out of the totalrequirement of fluxing agent added as hydroflux with 5-7% CaSO₄ cause asmall increase on the sulphur content in the steel

TABLE 23 Slag samples after reduction ASTM 304L (% by weight) Heat451132 851133 851134 851135 851136 851137 851138 851139 851140 ElementREF 11 Test 17 Test 18 Test 19 REF 12 Test 20 Test 21 Test 22 REF 13 S0.13 0.09 0.17 0.08 0.16 0.12 0.12 0.15 0.09 F 5.5 3.0 5.5 4.5 5.4 4.24.2 4.1 4.4 SiO2 30.0 34.3 31.1 33.2 30.9 33.7 34.3 33.2 33.1 MnO 0.51.4 0.8 1.3 0.3 1.1 1.4 1.0 0.5 P2O5 0 0 0 0 0 0 0 0 0 Cr2O3 0.6 0.8 0.60.9 0.6 0.8 0.7 0.6 0.7 NiO 0.05 0.07 0.11 0.06 0.04 0.05 0.05 0.05 0.05TiO2 0.25 0.37 0.25 0.27 0.23 0.21 0.19 0.24 0.15 V2O5 0.01 0.03 0.010.02 0.02 0.01 0.01 0.02 0.02 Al2O3 1.1 1.4 1.1 1.2 1.2 1.2 1.1 1.1 1.2CaF2 11.3 6.2 11.3 9.1 11.1 8.7 8.6 8.4 9.1 CaO 59.5 55.7 56.8 54.6 61.157.2 56.2 56.0 58.8 CaO calc 51.4 48.7 48.7 48.1 53.1 50.9 50.0 49.952.3 FeO 0.4 0.7 1.3 0.6 0.1 0.4 0.4 0.6 0.3 MgO 9.2 7.1 7.5 7.8 7.6 6.76.5 7.1 6.9 Basicity 1.7 1.5 1.6 1.5 1.7 1.5 1.5 1.5 1.6

The experiments showed that fines with particle sizes of max 1-2 mm orless have a tendency to be caught by the mixing gas that is blownthrough the heat and these fines are transported and deposited in thegas cleaning plant which is undesired. Suitably the hydroflux isscreened such that a hydroflux product with particle sizes of at least 2mm is obtained. The fines fraction can be formed to briquettes wherebythey can be used in the converter as well.

Results of the Evaluation of the Material Produced During 2^(nd) and3^(rd) Campaign

The test heats and the reference heats all have been compared in respectof weldability, surface quality, slag inclusions and strength accordingto the same procedure as in the initial full scale experiments. Not asingle defect has been observed.

Alternative Embodiments

In a preferred embodiment of the invention, an electrically heated,stationary furnace (of bell-type) is used for the calcination ofhydroxide sludge. Performed experiments have showed that this results ina product having very good properties in tenns of mechanical stability,sintering degree and piece/particle size. However, the invention is notlimited to this, but according to undertaken experiments, a Kaldoconverter can also be used for the calcination. The person skilled inthe art will realise that other types of equipments can be used too, forexample a belt furnace, an electrical or LP-gas or oil-heated stationaryor batch type furnace such as a tunnel kiln or a walking beam furnace, aCLU-, OBM- or LD-converter, enabling the production of a product havingthe desired properties.

In the pilot experiments, the hydroflux had a piece/particle size ofbetween 12-40 mm. These experiments were conducted without any dustingtendencies of the hydroflux at the handling after calcination and at therecycling to the AOD-converter. In the pilot experiments, the hydrofluxwas added via a material hopper above the AOD-converter that was in anupright position. It is also possible to add the hydroflux aftercharging of the steel, while the converter is still in tilted position.In the full scale experiments, hydroflux of different particle sizeshave been used. The experiments have shown that a hydroflux productaccording to the invention can be added to the AOD-converter inconventional manner, i.e. added from the material hoppers above theAOD-converter when the converter is in upright position. Suitably thehydroflux is screened such that a hydroflux product with particle sizesof at least 2 mm is obtained. The fines fraction can be formed tobriquettes whereby they can be used in the converter as well.

The concept of the invention also comprises use of the hydroflux productin other applications in which natural fluorspar is used, e.g. in theslag purification that is performed directly before continuous castingin a ladle furnace. In such an application, it is conceivable to allowthe hydroflux to have a particle size smaller than 10 mm, suitablysmaller than 5 mm, preferably 2-4 mm, and to add it via a squirt, to theladle furnace.

It is also realised that a calcined hydroflux that risks dusting or notto penetrate into the slag as is desired, can be packed in individualportions in order to allow handling and addition without said drawbacks.

In the experiments, the addition of the calcined hydroxide sludge hastaken place in connection with a reduction step in an AOD-converter. Theperson skilled in the art will however realise that the invention is notlimited thereto, but that the calcined hydroxide sludge can be used as afluxing agent in connection with a decarburization and/or reduction stepin some other equipment, such as a CLU-converter.

It is furthermore realised that the content of CaF₂ in the hydroflux canbe varied because the product can be used as a supplement to naturalfluorspar. In a preferred embodiment, the content of CaF₂ in thehydroflux is 40-65% by weight, but with the metal contaminated picklingagents occurring in various pickling plants as a starting point, it islikely that the content of CaF₂ will vary between 20 and 80% by weight.

According to a preferred embodiment, a hydroxide sludge is used that hasa natural low content of sulphur, in the form of calcium sulphate. Thesulphur content of a hydroxide sludge that has a natural low content ofsulphur is less than 0.1%. The invention is however not limited to theuse of essentially low sulphur containing hydroxide sludge. Hydroxidesludge containing amounts of sulphur, suitably less than 15%, e.g. inthe form of calcium sulphates, can also be used, as demonstrated in thefull scale tests. Such contents occur in hydroxide sludge fromproduction in an annealing and pickling line containing both a neolytepickling section and a mixed acid section and in hydroxide sludgeexisting in landfills of today. Given the experiments performedhitherto, it seems that sulphur does not significantly affect the steelproduction process, but a higher content >15% of calcium sulphates seemto result in a somewhat increased content of sulphur in the steel andthe slag. The main part of the sulphur leaves together with the effluentgases, as SO₂.

The major part of the hydroxide sludge produced in a neutralisationplant of the applicant originates from pickling liquids from the acidretardation plant, SAR, from spent pickling baths, and from neutralisedflushing water from the pickling baths. A minor amount, about 5-10%,originates from a chromium reduced electrolyte from a neolyte picklingstep, which is a pre-pickling method especially designed for cold-rolledsurfaces before pickling with mixed acid. The neolyte pickling stepcontains sodium sulphate (Na₂SO₄) as an electrolyte. In thisneutralisation plant the filtering of hydroxide sludge is howeverperformed in seasons, why it is possible to obtain a hydroxide sludgehaving low sulphur content. In the other neutralisation plants of theapplicant, all pickling liquids are mixed before they are led to theneutralisation plant, which results in a higher sulphur content in theproduced hydroxide sludge.

1-14. (canceled)
 15. A fluxing agent that can be used in production ofsteel, preferably stainless steel, produced by a method that comprises:providing a hydroxide sludge resulting from neutralisation ofmetal-contaminated pickling liquid from a pickling step for a steel,said hydroxide sludge containing at least one fluoride-containingcompound, heating said hydroxide sludge to a temperature from 1000° C.to 1300° C., allowing the hydroxide sludge to cool to ambienttemperature, and crushing the cooled hydroxide sludge.
 16. A fluxingagent according to claim 15, wherein it contains 20-80% by weight,preferably 40-65% by weight, of CaF₂.
 17. A fluxing agent according toclaim 15, wherein it contains residual oxides originating from metals inthe metal-contaminated pickling liquid.
 18. A fluxing agent according toclaim 15, wherein it contains (in % by weight): 20-30 Fe₂O₃ 4-10 Cr₂O₃1-4 NiO 8-12 CaO 1-3 SiO₂ 0.1-15 CaSO₄ 0.2-0.5 MnO 0.4-0.6 MgO max 0.02C.
 19. A fluxing agent according to claim 15, produced by a method inwhich the step of heating said hydroxide sludge comprises heating thehydroxide sludge to a temperature in the range 1000-1200° C., wherebythe hydroxide sludge is sintered into a mechanically stable product. 20.A fluxing agent according to claim 15, produced by a method in which thestep of heating said hydroxide sludge comprises heating the hydroxidesludge to a temperature at which the hydroxide sludge is sintered into amechanically stable product.
 21. A fluxing agent according to claim 15,produced by a method in which the step of heating the hydroxide sludgecomprises heating the hydroxide sludge to a temperature above 1200° C.,whereby the hydroxide sludge melts and subsequently solidifies oncooling to ambient temperature.
 22. A fluxing agent according to claim15, produced by a method in which the step of heating the hydroxidesludge comprises heating the hydroxide sludge to a temperature at whichthe hydroxide sludge melts, so that the molten hydroxide sludgesubsequently solidifies on cooling to ambient temperature.
 23. A methodin connection with steel production, preferably stainless steel,comprising producing a steel heat and decarburizing the steel heat,whereby a slag is formed on top of said steel heat, and adding a fluxingagent to said slag, said fluxing agent being produced by a methodcomprising: providing a hydroxide sludge resulting from neutralisationof metal-contaminated pickling liquid from a pickling step for a steeland containing at least one fluoride-containing compound, heating saidhydroxide sludge to a temperature from 1000° C. to 1300° C., allowingthe hydroxide sludge to cool to ambient temperature, and crushing thecooled hydroxide sludge.
 24. A method according to claim 23, comprisingadding said fluxing agent to the slag in an amount that partly ortotally corresponds to the requirement of CaF₂.
 25. A method accordingto claim 24, comprising adding said fluxing agent in an amount thatcorresponds to 30-70% of the requirement of CaF₂ in order to achieve adesired fluxing effect.
 26. A method according to claim 24, comprisingadding said fluxing agent preferably in an amount that corresponds to50% of the requirement of CaF₂ in order to achieve a desired fluxingeffect.
 27. A method according to claim 24, wherein the decarburizationis followed by a reduction step in which at least a part of the metalcontent in the fluxing agent is reduced and forced into the steel heatby an extra addition of FeSi.
 28. A method in connection with steelproduction according to claim 27, wherein the reduction step comprisesaddition of CaO.